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Is Air Quality Better or Worse?


Missouri has a notorious role in the annals of air quality. During the first part of the 20th Century smoke pollution had been a problem in St. Louis. In 1939 it was particularly bad, however. On November 28, 1939, a temperature inversion trapped pollutants; a thick cloud of dark smoke blanketed St. Louis, blotting out the sun. The day came to be known as “Black Tuesday,” or “The Day the Sun Didn’t Shine.” You can view photos by searching for “black tuesday St. Louis” on Goggle Images. It was one of the worst air quality events in recorded history.

Since then, many steps have been taken to reduce air pollution, and Black Tuesday events don’t occur any more. But how have we been doing recently? Is our air quality still improving, or has it peaked and started to backslide?

I haven’t been able to find a report that summarizes air quality data for the state. Since the 1980s, however, the EPA has gathered air quality data from cities and counties in Missouri and maintained it in a national database, and this data can be used to construct an analysis.

I downloaded data going back 10 years, from 2012 to 2003. In addition, to give a longer term perspective, I downloaded data for 1993 and 1983.

AQI CountiesNineteen counties have been monitored for more than 6 of the 12 years for which I downloaded data. A map showing their locations is at right. They can be gathered into three groups: a group along the Mississippi River, a group in the Kansas City-St. Joseph Area, and a widely dispersed group that does not fall into either of the other two.

Air quality tends to be a function of a number of pollutants that are emitted into the atmosphere: particulates, ozone, carbon monoxide, nitrous oxide, and sulphur dioxide are five of the most important.

Particulates are tiny particles of matter that float around in the atmosphere. When we breathe, we inhale them, and if there are too many of them, they cause lung damage. The small ones (less than 2.5 micrometers – 100 times thinner than a human hair) are the most dangerous.

Ozone is a highly corrosive form of oxygen. High in the atmosphere, we need ozone in order to absorb ultra-violet radiation. But at ground levels, too much of it can cause lung damage.

Nitrogen oxides react with ozone and sunlight to form smog, and they irritate the lungs when breathed.

Sulphur dioxide smells like rotten eggs. Too much of it causes lung damage, and it also reacts with water vapor in the atmosphere to form sulphuric acid, the main ingredient of acid rain.

The biggest sources of air pollution are power plants, industrial facilities, and cars. These tend to concentrate in urban areas, but air quality can be a concern anywhere. In addition, weather plays an important role in air quality. On some days, weather patterns allow pollution to disperse, but on others they trap it, causing air quality to worsen. Hot, sunny summer days are of particular concern, but unhealthy air quality can happen any time. Black Tuesday was in November, after all.

The EPA has established maximum levels of each pollutant, and reports the number of days on which there are violations. EPA also combines small particulate and ozone levels into an Air Quality Index, or AQI, in order to represent the overall healthfulness of the air. The AQI is a number, but it does not have an obvious meaning. Suppose the median AQI is 75 – what does that mean? So EPA has created six broad AQI ranges: Good, Moderate, Unhealthy for Sensitive Individuals, Unhealthy, Very Unhealthy, and Hazardous. EPA reports a yearly AQI number and the number of days in which the AQI falls in each range.

In following posts I will report on Missouri’s AQI, then on the specific pollutants that seem to cause repeated violations.


1939 St. Louis Smog, Wikipedia, http://en.wikipedia.org/wiki/1939_St._Louis_smog.

Air Quality Index Report, http://wwwepagov/airdata/ad_rep_aqihtml.

What Are the Six Common Air Pollutants? EPA, http://www.epa.gov/airquality/urbanair.


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